In Cosentino U.S. Pat. No. 4,068,521 an ultrasonic air and blood foam detector is disclosed, made of a two-piece housing which is clamped together about tubing in which bubbles are to be ultrasonically detected.
Cole U.S. Pat. No. 3,921,622 also discloses a two-piece housing for an ultrasonic bubble detector in which tubing is installed in the housing, and the two pieces of the housing are brought together to bring a pair of ultrasonic transducers into contact with the tubing.
In Miller U.S. Pat. No. 3,663,842, an ultrasonic probe is disclosed, having a composite matrix material as a device for coupling the signals from piezoelectric elements to the desired material for testing, in which the composite matrix may contain an elasotomeric component.
One problem with the construction of ultrasonic bubble detectors is that ultrasonic energy tends to find its way through the respective housings from one transducer to the other without passing through the tubing through which bubbles may pass. This provides an undesirable background noise level, which reduces the sensitivity of the apparatus.
Accordingly, it has been generally preferred for the various two-piece housings utilized in ultrasonic detectors to have an air gap between them, to block the transmission of ultrasonic energy by routes other than the desired route passing through the liquid-filled conduit to be monitored.
The existence of transmissions of ultrasound energy by routes other than that desired between the two transducers has tended to lead those skilled in the art away from the use of one-piece housings in accordance with this invention. Under the prior art conditions of use, such a one-piece housing could have a very high background noise, due to the transmission of ultrasound energy directly in the housing from one transducer to another.
It has also been found that when piezoelectric transducer sending and receiving crystals are of a thickness selected to be resonant at a frequency on the order of 1 to 3 MHz, the resulting ultrasonic vibration exhibits substantial advantages over lower frequency sonic systems, in that such high frequency sonic vibrations are very poorly coupled with air. Accordingly, the presence of a bubble can cause a very major change in the sensing signal, so that the ultrasonic bubble detection systems may have substantially increased sensitivity.